1. Field of the Invention
This invention relates to a controllable-frequency oscillator or an adjustable-frequency oscillator.
2. Description of the Related Art
A typical temperature-compensated crystal oscillator has an oscillating section and a temperature compensating section. The oscillating section includes a quartz-crystal resonator. The oscillating section also includes a circuit connected to the quartz-crystal resonator for generating a signal having a frequency which can be adjusted by a control signal in a range around the resonance frequency of the quartz-crystal resonator. The temperature compensating section includes a temperature sensor. The temperature compensating section also includes a circuit connected to the temperature sensor for generating a temperature-dependent signal. The temperature compensating section feeds the temperature-dependent signal to the oscillating section as a control signal in order to compensate for a temperature-dependent variation in the frequency of the signal generated by the oscillating section.
Japanese published unexamined patent application 6-276020 discloses a controllable-frequency crystal oscillator of the Colpitts type. The crystal oscillator in Japanese application 6-276020 includes only one variable-capacitance diode in a resonator. Furthermore, only one control signal can be applied to the variable-capacitance diode. The frequency of a signal generated by the crystal oscillator can be varied in response to the control signal applied to the variable-capacitance diode. In Japanese application 6-276020, the control signal is generated on the basis of the output signal of a temperature sensor to implement temperature compensation with respect to the frequency of the signal generated by the crystal oscillator.